The present invention relates to microwave fault locating systems and, more particularly, to an automated fault locating system employing a low-loss coupler to couple the test signal into and out of an aircraft antenna from outside of the aircraft and for displaying the location of a fault as a function of the portion of the circuit under test and the distance to the fault.
The description hereinafter is with respect to an aircraft test system. Those skilled in the art will recognize that the invention could also be used on ships, missiles, tanks, submarines, and the like. It is intended therefore that the scope of the description and claims which follow include these other applications despite the use of the word "aircraft".
Turning to FIG. 1, a portion of an aircraft 10 is shown containing a microwave system, generally indicated as 12. Microwave system 12 includes antennas 14, a divider 16, and a receiver/transmitter 18. The two antennas 14 are connected to divider 16 by transmission lines 20, having connectors 22 therein. The divider 16 is connected to the receiver/transmitter 18 by another transmission line 20 also containing connectors 22. Other transmission lines 20 to and from the receiver/transmitter 18 are connected to other elements (not shown). The above-described components in the aircraft 10 are contained behind a plurality of access doors in the aircraft's skin (not shown). Typically, problems are created by personnel opening the access doors and manipulating the transmission lines 20 and connectors 22. Connectors 22 are left disconnected or are broken causing improper connections such that the continuity between the antennas 14 and the receiver/transmitter 18 in the various segments of the total circuit of the microwave system 12 is impaired.
One method of testing the microwave system 12 is shown in FIGS. 2 and 3. A test unit 24 connected to a test antenna 26 by line 28 is used to insert a test signal into one of the antennas 14 from whence it travels down the associated transmission line 20, as indicated by the arrow 30, and is received by an antenna, similar to test antenna 26, disposed at an aircraft 10 transmitting antenna (not shown) connected to an opposite end of transmission line 20, whence it is provided to test unit 24. If a discontinuity occurs at one of the connectors 22 as shown in FIG. 3, the signal arriving at such transmitting antenna of aircraft 10 via transmission line 20 is changed. Alternatively, if test unit 24 is a reflectometer, then line 28 can be directly connected to transmission line 20 rather than through test antenna 26 and an antenna 14. In this alternate configuration, test unit 24 connected to transmission line 20 is used to insert a test signal into transmission line 20 which returns to line 28 and test unit 24 as a result signal. If a discontinuity occurs at one of the connectors 22 as shown in FIG. 3, then the return signal is changed and the test unit 24 can determine the distance along the transmission line 20 to the fault. Typically, this is followed by further opening of the access panels to search for the exact location of the fault since the distance to the fault is only an approximate indicator to the technician. Further access through the access panels often gives rise to further inducement of faults into the system 12.
As shown in FIGS. 2 and 3, the test antenna 26 is typically close coupled to the antenna 14. It would be desirable to completely eliminate the necessity for physical access to the system 12 but, unfortunately, with prior art systems, this has been impossible. The reason is shown in FIGS. 4 and 5. According to the prior art, it has been attempted to position the test antenna 26 within an enclosure 36 of an anechoic material to be placed against the fuselage 38 of the aircraft in alignment with the antenna 14 to be tested. The result, typically, is a 20 dB loss between the test antenna 26 and the antenna 14 under test. A 10 dB loss is typical between the antenna 14 and the receiver/transmitter unit under test (UUT) 18. With typical test apparatus available in the art, this two-way loss of 60 dB (20 dB+10 dB+10 dB+20 dB in two directions) is greater than allowable to still obtain meaningful results. This is as a result of the typical antenna pattern as shown in FIG. 5. Antenna 14 typically has a major lobe 14 disposed along its alignment axis 42 with a plurality of minor sidelobes 44. With an arrangement such as that of FIG. 4, a significant portion of the radiated energy to and from the antenna 14 is lost which accounts for the high loss at the interface.
Wherefore, it is the object of the present invention to provide a microwave fault locating system which incorporates a low-loss coupler and which provides direct indication of fault location whereby the necessity for physical access to the system through access panels other than at the known site of a fault to correct the fault is eliminated.
It is a further object to provide a test system for a vehicle microwave system wherein the determination and location of faults is accomplished without previous disassembly or teardown of the vehicle and microwave lines.